Bill Steele, seismologist and spokesman for the Pacific Northwest Seismic Network.
Bill Steele, seismologist and spokesman for the Pacific Northwest Seismic Network.

Local

Q&A: South Sounders should be prepared to go it alone in Cascadia earthquake

By Kate Martin

Staff writer

July 19, 2015 06:00 AM

The New Yorker had Pacific Northwest residents talking this week about “The Really Big One,” a story in the magazine’s latest edition that describes the devastating impacts of a full Cascadia subduction zone earthquake.

The story was a grim tale of tsunamis outrunning coastal dwellers’ ability to flee, of refrigerators walking out of kitchens and homes lurching off their foundations, of a region left unrecognizable and economically unviable, of nearly 13,000 people dead and 1 million displaced.

The News Tribune asked Bill Steele, a seismologist and spokesman for the Pacific Northwest Seismic Network, what South Sounders could expect if such an earthquake happened in the Cascadia subduction zone, which spans 700 miles from Vancouver Island in Canada to Mendocino County, California.

If the entire fault ruptures, as it last did in the year 1700, the earthquake’s magnitude could be between 8.7 and 9.3.

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Q: What will that feel like to people who live in Pierce County?

A: Pierce County was very lucky during the Nisqually Earthquake, much of it. It got very mild shaking — around 6 percent of the force of gravity. So maybe twice as strong as they experienced in the Nisqually Earthquake? So still not really strong shaking, but shaking enough to damage poorly built structures, perhaps induce liquefaction.

The big difference between the Nisqually Earthquake (which lasted 40 seconds) and the Cascadia subduction zone event is the duration. So, for a Cascadia earthquake you can expect two minutes of really strong shaking or strong shaking, and a few more minutes of very weak shaking as the rupture approaches the area. It will sort of build up. Altogether, you may feel it for four or five minutes. Only a couple of those minutes might be strong shaking, but that seems like forever when you’re in an earthquake.

Q: When the shaking starts, some of the soils will liquefy. How big would an earthquake have to be to cause liquefaction?

A: Generally it requires shaking to get up around 10 percent of the force of gravity, of horizontal shaking, and long enough duration to where the sand or soft soils can compact. And you need a water table there that’s fairly high so when that ground tries to compact, and the crystals align, the water doesn’t compress.

As you know, trying to put a cork in a bottle, the water doesn’t change volume when you do that, it just gets under higher pressure. So what happens is the water pressure in the water table shoots up very, very high. When it gets to a certain point it suspends the grains of sand, and then the whole soil column behaves kind of like a viscous liquid as opposed to a solid.

And I think for much of the Puget lowland and the I-5 corridor we expect the shaking to exceed that level (10 percent of the force of gravity).

Q: What would that look like?

A: As liquefaction initiates, you will see water erupting out of the ground and sometimes shooting up meters into the air in sand volcanoes, where sand and water shoot up from depth and kind of cover the surface of the land. And you end up with a shallow flood, essentially.

During the 1949 earthquake and 1965 earthquake, the whole Puyallup valley flooded with liquefaction; and it’s not because the river leaked through the levees, it was because the groundwater erupted onto the surface. It just looked like a massive flood throughout Puyallup and parts of Tacoma.

Q: What do you think is the biggest threat to people who live in Pierce County from a Cascadia fault earthquake?

A: I would say the shaking initially is really the biggest single thing. The liquefaction and ground failure in general will also be a big factor. Big block landslides into deep water can generate tsunamis as Gig Harbor found out after the 1949 earthquake, when part of the bluffs above Salmon Beach gave way, fell into the Sound and produced a tsunami that affected the Gig Harbor area.

It’s a combination of the direct shaking and the number of unreinforced masonry structures and poorly built structures that are very vulnerable to this. It’s going to cause some buildings to collapse and that’s probably going to take all of our emergency services to deal with, and then all of the other problems that are coming up. There’s going to be inadequate first responders to deal with them.

Neighborhoods are going to be on their own for a good week or two. That’s one of the other real challenges to remember. People’s homes are likely to ride through these earthquakes, even the magnitude 9. There may be some damage to a chimney or something. But the pharmacy is not going to be there for you and the grocery store is not going to be there for you. You're not going to be able to drive around. And communication will be difficult.

Q: The New Yorker story talks about a giant tsunami that would overwhelm the coastal areas. How would Puget Sound fare in the aftermath of such an earthquake?

A: There is an uncertainty there as well. We are not going to get a lot of the energy of the direct tsunami that hits the outer coast. When it hits the coast of North America, there’s a little tiny opening at the Columbia River. Some of the energy will be rolling up there and may affect the Portland area.

There’s also a larger opening at the Strait of Juan de Fuca. It’s deep water there, so the tsunami will come in there. But then it’s going to hit the coast of Vancouver Island and it will affect Port Angeles and some of the towns along the Strait. Then it hits the Puget Sound, opens up again, so what’s left of that energy will then be distributed further.

We are likely to see variation in the sea level of the Sound of 1-3 feet or so. But we are going to have unusually strong currents.With any big earthquake, it’s good to get away from the water because the shaking can break loose a large bluff or landslide that can drop into the Sound anywhere and generate a locally pretty intense wave.

So there’s no real safe place down by the water after a big earthquake.

Q: Which city would you rather be in during the big one: Seattle, Tacoma or Olympia? And why?

A: That’s a great question. Probably Seattle because I live here and I know it better. I feel pretty confident I can navigate. I can get a kayak in the water, and if the bridge is down, be able to move around. But as far as impacts go, I think we are all fairly equally exposed. We all have large number of these very dangerous, unreinforced masonry buildings, and none of us have come up with a good way of getting them either retrofitted or removed.

Q: What else should people consider when living in a beautiful area known for massive earthquakes?

A: Just that we need to be a little bit responsible. Again, if you lived out in the Alaska wilderness somewhere in a gorgeous place, you would know that you have to be able to have enough fuel to keep warm in the winter. You don’t go out without your rifle just in case the grizzly bears want to eat you or whatever. There are certain precautions you take.

There are some even simpler precautions that we need to make, and that’s put away food and water and medicine to take care of our family and pets for a week or two. That’s not a difficult thing to do. It’s not a real expensive thing to do, but it does take a little bit of forethought, a little bit of action, and setting aside a little bit of room to stack some water up.

I just encourage people not to so much worry about big earthquakes in the future, because they are going to happen when they happen, but to do one thing, do one or two things today or this week to make your family better prepared to ride out a week or two without services.

Q: Was there anything else you’d like to add?

A: I think that the New Yorker article was beautiful in some ways and frightening in some ways, and it was nice to look at some of the history of some of the great scientists like Brian Atwater and Chris Goldfinger and others that have been exploring the Cascadia subduction zone. But it was a little bit doom and gloomy.

The fact of the matter is when you walk through a residential neighborhood in Tacoma or in Puyallup, you are going to see the houses looking pretty good after this Cascadia earthquake, maybe some toppled chimneys. You and your neighbors are going to be around. How comfortable that next couple of weeks are — is it an adventure or is it a real tragedy — depends on what we do now to prepare.

Get prepared

Emergency planners say Northwest residents should have on hand seven to 10 days of supplies to help them ride out the aftermath of a major earthquake.

An emergency kit should include:

▪ 1 gallon per person per day for drinking and sanitation

▪ Nonperishable food

▪ Cash, because ATMs won’t work without electricity. Small bills are best.

▪ Battery-powered radio and extra batteries

▪ Flashlight and extra batteries

▪ First aid kit

▪ Whistle to signal for help

▪ Filter mask or cotton T-shirt to help filter the air

▪ Wrench or pliers to turn off utilities if needed

▪ Shelter items like tents, tarps and rope

For more ideas of what to include and how to prepare, go to makeitthrough.org.